1. Field of the Invention
The present invention relates to the art of biocompatible oxygen transport and contrast enhancement agents for animal use, and more particularly to biocompatibly fluid fluorocarbon containing emulsions having high fluorocarbon concentrations and substantial stability.
2. Description of the Prior Art
In the past, fluorocarbons in aqueous emulsions with an emulsifying agent have been known for medical applications involving animals, including humans, for radiopacity and oxygen delivery. Oxygen and gases in general are highly soluble in some fluorocarbons. For example, see Long, U.S. Pat. No. 3,818,229; No. 3,975,512; and, No. 4,073,879.
Efforts to use emulsified fluorocarbons as an oxygen carrier, as in a blood substitute, have encountered certain difficulties. Purity, non-toxicity, chemical and biological inertness and excretability of the ingredients, especially the fluorocarbons and any fluorocarbon emulsifying agents, as well as a high fluorocarbon concentration in the emulsion are desired objectives. The emulsion must be capable of sterilization, preferably by heat, have long-term particle size and function stability in the fluid or non-frozen state, preferably at ambient or room temperatures, be industrially feasible or capable of manufacture on a large scale, persist for sufficiently long times in the body or blood stream when used intravascularly, be eliminated rapidly from the body thereafter and have a high enough concentration of the fluorocarbon while remaining biocompatibly fluid to be effective, whether as a contrast enhancement agent or as an oxygen carrier.
For intravenous and vascular uses, it is considered important to have small particle size. However, long term storage for extended periods of time for a month or longer, of fluorocarbon containing emulsions acceptable for intravascular use, i.e. with fluorocarbons having half retention times in the organs of approximately seven days or less such as is desirable for blood substitutes or "synthetic blood", has heretofore resulted in coalescence or conglomeration of the fluorocarbon particles in the emulsion into larger particles, especially during and after heat sterilization. For a general discussion of the objectives and a review of the efforts and problems in achieving these objectives in fluorocarbon blood substitutes, see "Reassessment of Criteria for the Selection of Perfluorochemicals for Second-Generation Blood Substitutes: Analysis of Structure/Property Relationship" by Jean G. Riess, 8 Artificial Organs, 34-56 (1984).
Particle sizes significantly larger than 0.4 micrometers (microns) tend to occlude small vessels and to collect too rapidly in the liver, spleen and some other organs, enlarging them and endangering their functioning. On the other hand, it is desired in certain applications to have sufficient emulsion particle size in order that the particles will collect around and in tumors, abscesses and infarcted myocardial tissue and infarctions in other tissues, and in the liver, spleen and bone marrow when fluorocarbons are used as a contrast enhancement medium. Larger particle sizes are unobjectionable when used in other, non-venous systems in the body, such as, for example, the trachea, the cerebrospinal fluid ventricles and other cavities of the body.
Herein in this specification, the term "weight per volume," "% weight per volume", "w/v" or "% w/v" will be used and should be understood to mean that ratio which is the weight in grams per 100 cubic centimeters or 100 milliliters, or equivalent expressions or mathematical identities thereof. This definition is meant to be consistent with that used in the field and which is given in, for example, the herein referenced patent to Sloviter. Thus, for example, an emulsion having a "5% w/v" of an ingredient has 5 grams of that ingredient per 100 ml of the final emulsion.
In the past, fluorocarbon emulsions particularly formulated for oxygen carriage have been taught to have upper limits on the fluorocarbon concentration. For example, efforts directed toward perfluorocarbon emulsions with phospholipid emulsifiers have been proposed having 20% to 40% weight per volume of the fluorocarbon and 2% to 6% weight per volume of lecithin, but such emulsions have a limited stability. Moreover, it has been taught that emulsions having fluorocarbon concentrations higher than 75% weight per volume are too viscous to be used intravascularly. See, for example, Sloviter, U.S. Pat. No. 4,423,077. Such concentrations, however, necessarily limit the capacity of the emulsion and the quantity of oxygen and of contrast enhancement which the emulsion can provide.
Moreover, the methods taught for achieving emulsions having higher fluorocarbon concentrations, on the order of from 50% to 75% weight per volume, required sonication for the turbulence or homogenization, and emulsification steps. Such methods, however, significantly limit the manufacturing or fabricating capability, for the quantities capable of fabrication using sonication are severely limited, and not considered of an industrial or commercial scale.
Moreover, it has been considered that higher weight per volume fluorocarbon concentrations were more toxic, apparently because it is more and too viscous. See, for example, Riess, cited above, 8 Artificial Organs (1984), at 49 where a 38% increase in mortality (50% versus 20%) is reported with the more concentrated 35% w/v fluorocarbon in emulsion than with the 20% w/v fluorocarbon in emulsion.
However, for many applications, such as percutaneous transluminal coronary angioplasty (PTCA), cerebral ischemia, organ preservation, myocardial infarction, as an adjunct to cancer radiation and chemotherapy and the like, higher fluorocarbon concentrations are desired for their higher oxygen dissolving capacity. The higher the concentration of the fluorocarbon, the less quantity of the emulsion that is needed to achieve the requisite contrast enhancement or quantity of oxygen to be administered. It is a medical desire and objective to minimize the total volume of medicines administered or inserted in a body. Further, it is desired to have higher concentration of oxygen to decrease the effects of hypoxia during ionic radiation treatments as described, for example, in J. C. Mottram, 8 British Journal of Radiology, at 32 (1935).
In other fluorocarbon emulsions, sterilization can only take place without damage to the emulsion, at temperatures lower than 121 degrees Centigrade (C.), on the order of, for example, 60 degrees C., and with repeated heatings. Many of these emulsions, further, must be stored frozen and thawed shortly before use, thus restricting handling and uses. Indeed, even in those emulsions previously taught as being sterilized at normal sterilizing temperatures, the desired emulsion is not obtained until centrifuging at 4 degrees C. at 100 times gravity for some period of time. See Sloviter, U.S. Pat. No. 4,423,077, mentioned above.
It should be noted, moreover, that the fluorocarbon F-decalin alone has heretofore been believed inconsistent or unstable in emulsion. This observation was made at fluorocarbon concentrations of 20% w/v, and it has been considered essential in order to achieve stability to add another fluorocarbon, such as F-tripropylamine, as in Fluosol-DC a trade name for a blend of fluorocarbons in emulsion. It is believed that such a combination of fluorocarbons, however, merely cumulates the disadvantages of the individual fluorocarbon components in the resultant emulsion, and is generally undesirable. For example, notwithstanding the Fluosol-DA combination, the F-tripropylamine continues to have long retention times in the body. Moreover, Fluosol-DA must be stored in substantially the frozen state. Another effort to solve the problem of stability in using F-decalin was to increase the lecithin concentration to at least 7% weight per volume, and as high as 9% w/v. See Sloviter, mentioned hereinabove.
It has generally been thought that to increase fluorocarbon concentration in an emulsion, the concentration of the emulsifying agent must also be increased. Thus, the weight per volume of lecithin, when used as the emulsifying agent, must be increased in order to increase the concentration of fluorocarbon in the emulsion. See Sloviter, mentioned above, who describes a ratio of fluorocarbon to emulsifying agent in the emulsion of at most 10.7:1 by weight per volume, i.e. the emulsifying agent concentration is approximately 9.2% of that of the fluorocarbon concentration in emulsion.
Where yolk lecithin, a frequently chosen emulsifying agent because of its known biocompatibility, is used, the emulsion is subject to degradation in the presence of oxygen. Oxygen attacks normally available lecithin, such as yolk lecithin, to oxidize the lecithin molecule which may result in a possible introduction of toxicity and degradation of the emulsion. Thus, in the presence of oxygen, the pH of the emulsion decreases due to the accumulation of carbon dioxide and fatty acids, and the pO.sub.2 pressure of the emulsion decreases. For this reason, it has been generally considered important to store such emulsions under or sparged with nitrogen which is believed to be inert with respect to the emulsion.
Yolk lecithin, as well as other lecithins have fatty acids characterized by one or more carbon-carbon double bonds. These double bonds are vulnerable to oxidation, leading to production of free fatty acids and other products. The lecithin thus changes into toxic components including fatty acids and lysolecithin which may produce adverse effects or toxicity. Over time, the oxygen dissolved in the fluorocarbon particle provides such an attack. To avoid such an attack, many such fluids are sparged with nitrogen and kept substantially oxygen free until use. A Pluronic, such as Pluronic F-68 is an emulsifying agent normally less sensitive to oxidation, but may cause undesirable reactions in some intravascular applications.
Oxygenation of the emulsion when used intravascularly in the body naturally occurs through the lungs. For many applications, however, such as percutaneous transluminal coronary angioplasty (PTCA), stroke therapy and organ preservation, it is highly desirable that the emulsion contain adequate oxygen prior to use or application. Not only when used as an oxygen carrier, but also in contrast enhancement applications where oxygen delivery to the contrasted body part also is desired, such as, for an example, in PTCA and stroke therapy through the cerebrospinal space, fluorocarbon emulsions are oxygenated in order to increase the amount of oxygen carried and delivered. The higher that the concentration of oxygen being carried in the emulsion is, the less of the emulsion that will be necessary to achieve the desired oxygen related objectives. Since excess volume alone may present biocompatibility problems, limitation of volume injected by attaining high fluorocarbon concentration in the emulsion is a desired objective.
When used for oxygen carriage or transport, fluorocarbon emulsions which cannot maintain substantially consistent partial oxygen pressure (pO.sub.2) through sterilization, storage, processing and administration must be oxygenated immediately prior to use.
Frequently, however, it is desired to use fully oxygenated fluorocarbon emulsions at locations where oxygenation cannot be performed or is inconvenient, such as, for example, when treating myocardial tissue and other diseased tissue, when used as an adjunct with percutaneous transluminal coronary angioplasty, or when applying the emulsion at remote locations or in ambulances or field hospitals. Moreover, it is believed that some oxygenators de-stablize emulsions or catalyze emulsion breakdown. See, for example, Moore, et al., U.S. Pat. No. 4,105,798.
It is desired to provide a more uniform and more reliable pre-oxygenated, hence more immediately efficacious emulsion by performing the oxygenation during or shortly after the emulsion preparation before extended storage. Partial oxygen pressure (pO.sub.2) and pH maintenance and stability during and through heat sterilization, and through extended time storage, preferably at room or ambient temperatures tend to indicate that there is no oxidation or degradation of the emulsion. It is a desired objective, therefore, to provide a biocompatible fluorocarbon emulsion which maintains pO.sub.2 and pH during sterilization procedures-and during extended periods of storage.
It is desired also to provide methods of oxygenating biocompatible fluorocarbons during or shortly after manufacture or fabrication.
It is desired further to provide fluorocarbon emulsions having a higher concentration of fluorocarbon in emulsion. It is desired yet further to provide such high fluorocarbon concentrations in emulsion with less concentrations of emulsifying agents, yet having biocompatibly satisfactory fluidity, i.e. biocompatibly low viscosity.
It is additionally desired to have methods of preparing and formulating high fluorocarbon concentrations with relatively low emulsifying agent concentrations in emulsion which do not have physical or practical commercial limitations affecting the quantity manufactured.